Currently, distance measurement instrument can be divided into contact type and non-contact type. The contact type distance measurement instrument generally uses a traditional distance measurement technology, such as a coordinate measuring machine (CMM). Although the contact type distance measurement technology is quite accurate, because the object must be contacted, which may lead to damage to the object by a probe of the distance measurement instrument. Therefore, the contact type distance measurement instrument does not apply to the object with high cost.
Compared with the traditional contact type distance measurement instrument, operation frequency of the non-contact type distance measurement instrument may be higher than million frequencies, so it has been used in many fields. Non-contact type distance measurement technology can be divided into active type and passive type. The active non-contact type distance measurement technology projects an energy wave to the object and then calculates the distance between the object and the reference point according to reflection of the energy wave. Common energy wave includes visible light, high-energy light beam, ultrasound, and X-ray.